1. Field of the Invention
The invention relates to wavemeters for monitoring a wavelength of emission of a tunable laser, and particularly to controlling temperature dependent offset of measured wavelengths.
2. Discussion of the Related Art
Narrow band ArF and KrF excimer lasers (xcex=193 nm, 248 nm, respectively) and the molecular fluorine (F2) laser (xcex=157 nm) are used for photolithographic applications involved in the production of integrated circuits. Excimer laser radiation is typically used for producing structures in the dimensional range of 0.18-0.25 nm for the KrF laser, or between around 0.13-0.18 nm for the ArF laser. The 157 nm radiation of the F2-laser) may be used for producing feature sizes of  less than 0.13 nm.
Achromatic imaging optics are difficult to produce for this wavelength region. For this reason, radiation of narrow bandwidth is used to reduce imaging errors caused by chromatic aberations in the imaging optics. Typical acceptable bandwidths are in the range of 0.3-0.6 pm.
The narrowed laser emissions of excimer lasers are typically tunable within their broadband characteristic emission spectra. The emission wavelengths of these lasers is desired to be detected and controlled typically with an accuracy xc2x10.05 pm.
To this aim, a monitor etalon may be sealed off inside a housing of an etalon spectrometer arrangement for detecting and controlling the laser emission wavelength in conjunction with an absolute wavelength calibration procedure using a known absorption or emission line. The housing may be an evacuated housing (U.S. Pat. No. 5,025,445). A good vacuum is, however, difficult to maintain and such an evacuated housing typically involves using materials having low outgassing characteristics.
U.S. Pat. No. 5,420,877 discloses to periodically measure the temperature inside of a sealed etalon spectrometer housing using a sensor and a microcomputer to correct the spectrometer output according to formula (1):
xcexcorr=xcexetalon+k1xcex94T+k2xcex94T/xcex94txe2x80x83xe2x80x83(1)
where xcex94T is a difference between the measured temperature and a nominal temperature at which the wavemeter needs no correction; k1 is a coefficient of temperature sensitivity of the etalon and housing; k2 is a coefficient of sensitivity of the etalon to the time rate of change of the temperature (xcex94T/xcex94t).
The etalon wavemeter may be placed in a temperature stabilized enclosure (e.g., an oven). The U.S. Pat. No. 5,420,877 discloses that this typically involves a comparatively long time from the cold start of the laser before a desired wavelength accuracy and stability may be achieved.
In a sealed housing filled with a gas (e.g., nitrogen), the total mass of the gas within the housing is fixed. It is recognized in the present invention that there are at least three effects that will cause changes in a measured interference pattern from a monitor etalon, which pattern is used to determine the wavelength of incident radiation:
1. Since the internal volume of the housing varies with the temperature (xcex94V/V=3xcex1h xcex94T, xcex1h being the linear expansion coefficient of the housing material), the gas density varies inversely with the volume V, which effects the output pattern of the etalon.
2. Since the output pattern of the etalon also varies with the gap spacing between its plates, expansion of etalon spacers due to temperature variations (xcex94xcex4/xcex4=xcex1s xcex94T, xcex4 being the spacer thickness, xcex1s being the linear expansion coefficient of the spacer material) will effect the etalon output pattern.
3. If the temperature is not uniform within the housing, as can be the case when temperatures are rising and falling, the internal gas density xcfx81xe2x88x9d1/T will be spatially and temporally dependent, again effecting the output pattern of the etalon.
It is an object of the invention to provide a wavemeter having high thermal stability.
It is a further object of the invention to provide a wavemeter wherein thermal variations of measured wavelengths from actual wavelengths are minimized, such as preferably within xc2x10.1 pm/xc2x0K.
In accord with the above objects, a wavemeter for monitoring a wavelength of emission from a tunable laser is provided including a spectrometer disposed within a housing having a controlled pressure, and a temperature sensor for sensing the temperature within the housing, and a heater for adjusting the temperature within the housing, wherein the temperature is maintained approximately around a predetermined value, which value is selected based at least in part on a pressure within the housing, for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.
In further accord with the above objects, a wavemeter for monitoring a wavelength of emission from a tunable laser is provided including a spectrometer disposed within the housing having a controlled pressure, and a temperature controller for maintaining a temperature within the housing at approximately around a predetermined value, which value is selected based at least in part on a pressure within the housing, for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.
In further accord with the above objects, a wavemeter for monitoring a wavelength of emission from a tunable laser is provided including a spectrometer disposed within a housing having at least one port for controlling a pressure therein, and a pressure sensor, wherein a pressure within the housing is controlled at a predetermined value which is selected based at least in part on a temperature within said housing, for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.
In further accord with the above objects, a wavemeter for monitoring a wavelength of emission from a tunable laser is provided including a spectrometer disposed within a housing, wherein at least one of a temperature and a pressure within the housing is preset and substantially maintained at a predetermined value with respect to at least one of the pressure and the temperature, respectively, for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.
In further accord with the above objects, a method is provided for monitoring a wavelength of emission from a tunable laser, using a wavemeter including a spectrometer. The method includes disposing the spectrometer within a housing having a controlled pressure, sensing a temperature at least indicative of a temperature within the housing proximate to the spectrometer, and controlling the temperature within the housing approximately around a predetermined value, which value is selected based at least in part on a pressure within the housing, for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.
In further accord with the above objects, a method is provided for monitoring a wavelength of emission from a tunable laser, using a wavemeter including a spectrometer. The method includes disposing the spectrometer within a housing having a controlled pressure, sensing a pressure at least indicative of a pressure within the housing proximate to said spectrometer, and controlling the pressure within the housing approximately around a predetermined value which is selected based at least in part on a temperature within the housing, for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.
In further accord with the above objects, a method is provided for monitoring a wavelength of emission from a tunable laser, using a wavemeter including a spectrometer. The method includes disposing the spectrometer within a housing having a controlled pressure, and presetting and substantially maintaining a temperature and a pressure within the housing at substantially predetermined values for providing a temperature sensitivity of the spectrometer within xc2x10.1 pm/xc2x0K.